The present invention relates to an apparatus for the conversion of solar energy for heating and cooling by taking advantage of the large variation of the sorption capacity of molecular sieve zeolite. In particular, the apparatus relates to an integrated solar collector which includes a contiguous hermetically sealed zeolite panel and combined evaporator-condenser heat exchanger which converts small variations in absolute temperature of a water refrigerant under partial vacuum to relatively large variations in vapor pressure which, in turn, is utilized to produce cooling and refrigeration.
One of the primary difficulties which hinders use of solar energy for heat and cooling purposes is its low energy density (less than 1.5 kilowatt per square meter). The temperature differentials obtained with solar energy collectors are small and even when solar concentrators are used, temperatures above 400.degree.-600.degree. F. require sophisticated sun-following techniques. Thus, a need exists for apparatus which will efficiently convert solar energy to other forms of energy at small temperature differentials of, say between 50.degree.-180.degree. F. It has been found the unique characteristics of zeolites permit the design of such systems, especially to satisfy the needs for home heating, cooling and air-conditioning. The output of such systems increases as the solar load increases and therefore the higher needs for heating and cooling automatically are met by the higher output of such systems.
Those skilled in the art understand that due to the low temperature differentials obtainable with solar energy, Carnot efficiency of any system using the normal expansion of gases is of necessity quite low. For this reason, most solar energy refrigeration systems have concentrated on the known, dependable absorption refrigeration cycle based on the change of the solubility of a gas in a liquid with temperature. Inasmuch as this process is thermally activated, its dependence on temperature is exponential which permits large changes of gas pressure for small changes in absolute temperature. This process has received new impetus by commercial use of systems other than the ammonia-water used in early gas refrigerators. For example, at Kennedy Airport, New York City, an air conditioning system is provided which utilizes lithium bromide and water as working fluids.
In all refrigeration solid adsorption systems which have operated successfully, the heat source, supplied usually by a gas flame or steam, has been about 300.degree. F. Although such systems operate efficiently and with adequate capacity, none has achieved commercial importance. In contrast, solar heat from flat plate collectors rarely exceeds 190.degree. F. and the heat collection efficiency of the collectors is much higher at lower temperatures of 120.degree. to 140.degree. F. Due to the lower range temperature involved and, in particular, the reduced heat available from solar energy as a heat source, concentrated research and development efforts in the last few years, funded both by the Government and private industry, have failed to produce a cooling system which holds commercial promise. For example, modification of a Lithium Bromide system for solar energy has resulted in a drastically reduced capacity and low efficiency, requiring 80.degree. F. water cooler condensers. When the condenser temperature raises to 120.degree. F., as is necessary for air cooled condensers, a driving temperature at 140.degree. to 160.degree. F., which is reasonably obtainable from flat plate collectors, is insufficient for the system to operate.
Molecular sieve zeolites are a class of synthetic or natural mineral materials which have unique, non-linear adsorption properties described by exponentials to the second, third and fourth power in temperature and pressure. Zeolites have been found uniquely capable of converting small temperature differences into very large pressure differentials which can be practically utilized for both heating and cooling cycles. Zeolites also lend themselves to unique designs which utilize solid materials and diffusion through them to produce a solar refrigeration system of high conversion efficiency without moving parts which is, therefore, capable of long life and reliability.
Zeolites at room temperature absorb large quantities (up to 40% by weight) of any polar gas, that is gas with dipolar or quadropolar moment, such as H.sub.2 O, NH.sub.3, H.sub.2 S, N.sub.2, CO.sub.2, etc., as well as both fluoro-, chloro- and hydrocarbons. Due to the high non-linearity of their adsorption properties, zeolites adsorb large quantities of such polar gases when heated to temperatures which are easily achieved by flat plate solar collectors.
In practice, it was found that water vapor which was equiliberated at room temperature and had a partial pressure of 0.05 psia would have a pressure of 1.5 psia at 120.degree. F. Further, this temperature was sufficient to cause some water vapor to be desorbed from the zeolite and also condensed in a condenser held at 120.degree. F. By increasing the zeolite temperature to 140.degree. F., up to 10% by weight of the water vapor can be desorbed from the zeolite.
In contrast, other solid adsorbents such as silica gel, activated alumina and activated carbon adsorb much smaller quantities of such gases under the same conditions and desorb even less when heated to the 160.degree. to 200.degree. F. range. Thus, the resulting pressures are much smaller and the quantities of gas desorbed at high pressures are negligibly small. It has been found that liquid-gas systems suffer from the same shortcomings and do not operate efficiently, if they operate at all, at such low temperatures and high pressures. This is confirmed when driven by 140.degree. to 160.degree. F. with an air cooled condenser of 100.degree. to 120.degree. F.
In theory, the amount of adsorbed gas in a molecular sieve zeolite is represented by the equation EQU a=a.sub.o.sbsb.2 .theta..sub.2 +a.sub.o.sbsb.n .theta..sub.n
where a.sub.o is the limiting adsorption value of the gas and .theta..sub.n =exp- [(RTln(p.sub.s /p)/E.sub.n ].sup.n and n is an integer between 2 and 5. R is the universal gas constant; p.sub.s is the limiting saturation pressure; p is the actual pressure; and E.sub.n is the activation energy, which is on the order of a few kilocalories per mole. In this connection, reference is made to M. Dubin and V. Astakhov, "Description of Adsorption Equilibria of Vapors on Zeolites Over Wide Ranges of Temperature and Pressure," Second International Conference on Molecular Sieve Zeolites, Sept. 8-11, 1970, Worcester Polytechnic Institute, Worcester, Mass., pp. 155-166.
In view of the foregoing, it will be understood the adsorption process in molecular sieve zeolites is extremely temperature sensitive within a rather narrow range of temperature which is not far above room temperature. Also, zeolites are chemically inert, abundant and inexpensive.
A norm for solar energy collectors is the flat plate collector which has been known for many years. Such collectors are typically well-insulated and have a metal plate painted or plated with black which receives the solar radiation whereby about 90% of such radiation is absorbed and converted to heat. Due to the area available for solar collectors and necessary economies, the efficiency of a collector using adsorption for heating and cooling purposes should not be significantly greater or less efficient than a flat plate solar collector and its cost should not exceed that of a flat plate solar collector by an amount which exceeds the value of the cooling obtained.
For specific prior patents which disclose the state of the art, attention is invited to the following U.S. patents:
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